She didn't even want to do it. She's basically a jeans-and-backpack sort of person, her mother says. But to please her father, Lydia Monice Jackson entered the Miss Black America contest in her hometown, Willingboro, N.J.

So now she's Miss Black America 1978.

"I hated the idea of the bathing-suit thing," she said yesterday as she finished a TV taping session in Alexandria. "But as I'm planning to go into the performing field as an opera singer, I thought it would be a perfect opportunity to get the experience of working with large audiences and all that."

It started when her father, Russel, who is desegregation director at Rider College in New Jersey, met a friend in the local Kinsmen Club, a black men's service organization which sponsored the pageant this year in Burlington County. He talked the 20-year-old Lydia into trying out.

"Do it for me," he said. "It won't be so bad."

And it wasn't, at that. Once she saw that all the other girls were going along with the swimsuit contest, which isn't emphasized by the pageant anyway, it was easy. She took to a diet, lost eight pounds (not that she needed to), wowed everyone with "Uno Voce Poco Fa" from "The Barber of Seville" in its original mezzosoprano range.

"Eightteen of the 30 finalists sang pop songs," she said, "so this really made an impression."

A third-year student at Peabody Conservatory in Baltimore, she has trained as a singer more or less all her life. Her father is a tenor, her mother Betty an alto, her 18-year-old brother a basso and her 10-year-old brother a soprano at the moment. She started with the violin in third grade, but it wasn't until her last year of high school that she began to think seriously about singing.

Even that isn't the end of her ambitions.

I've always wanted to study international law. I'd love to be a diplomat. I like the different cultures, the languages, the travel."

She's studying French, German and Italian, not to mention political science.

But this year is special. This year she is takin off to enjoy the excitement and the perks and the $5,000-minimum worth of what promoters love to call personal appearances.Her mother is enjoying it all too. In the last few days they've seen the Bullets and the Caps, the Marine Marathon (Lydia sang the National Anthem there, and in January Miss Black America will tour our bases in the Pacific for the USO.

Yesterday she shot TV spots and recorded radio spots for the Armed Forces Radio and Television Service, and later this fall she will join pageant runners-up in rehearsing for the six-week tour.

"It's a much-sought-after trip," remarked Phillip Gaffin of USO. "The training is terrific for performers. You learn to handle crowds, to perform anywhere, on the back of a flat-bed truck, or on a carrier deck before an audience of 4,000 woman-hungry young men."

But why Miss Black America? Why not try for Miss America herself, as black women are doing more frequently these days? For Lydia Jackson, the reason is simply that the opportunity was there. For J. Morris Anderson, president of the Miss Black America organization in Philadelphia, the reasons go deeper.

"We started in 1968," he said, "as a positive protest against the limited standards of beauty represented by the other contests. If the ideal has got to be blond and blue-eyed, then black people get the idea they're not beautiful. We feel a black person should have the same right to say, 'I'm beautiful!'"

Emphasizing the mental and spiritual as well as the physical, the pageant gives all entrants "positivty training." They take an Oath of Positivity, and they get Success Secrets Seminars, which have proved so popular that they are about to be offered to the public, starting Nov. 29 in Philadelphia, by Fashionable Productions, the national sponsor.

"It worked so well" he maintaned, "the girls stopped worrying about who would win." "We see the contest as providing a stage for the black woman, a place to generate pride in the black man for his woman."

Lydia Jackson, smiling quietly with the poise of someone who knows where she is going, said this: "I'm glad to be representing black women. I'd like to be representing all women, as in Miss America. But I'm not trying to move mountains."